Here are 12 differeny mobs you can't spawn with eggs
1. Lightning Bolt] - /summon LightningBolt 
2. Giant] - /summon Giant 
3. Horses skeleton-zombie] /summon EntityHorse ~ ~ ~ {Type:3} & Skeleton /summon EntityHorse ~ ~ ~ {Type:4} 
4. Rabbit named] "Toast" - /summon Rabbit ~ ~ ~ {CustomName:Toast} 
5. Upside Down mobs] /summon Pig ~ ~ ~ {CustomName:Dinnerbone} - /summon Cow ~ ~ ~ {CustomName:Grumm} 
6. Techno Colored Sheep] - /summon Sheep ~ ~ ~ {CustomName:jeb_}
7. MinecartSpawner & MinecartCommandBlock] - /summon MinecartSpawner & /summon MinecartCommandBlock
8. The Killer Rabbit] - /summon Rabbit ~ ~ ~ {RabbitType:99}
9. Giant Wither] - /summon WitherBoss ~ ~ ~ {Invul:25000,CustomName:Grumm}
10. Giant Slime & Magmacube] - /summon Slime ~ ~ ~ {Size:20} & /summon LavaSlime ~ ~ ~ {Size:20}  (Can be any size)
11. Green robe villager] - /summon Villager ~ ~ ~ {Proffesion:5}
12. Broken Horse] - /summon EntityHorse ~ ~ ~ {Age:-50000}